Referring to FIG. 1, a conventional twin-spooled, contra-rotating propeller gas turbine engine, e.g. a “propfan”, “turboprop” or “open rotor” engine, is generally indicated at 10 and has a principal rotational axis 9 and an axial flow direction X in use. The engine 10 comprises a core engine 11 having, in axial flow series, an air intake 12, a low pressure compressor 14, a high pressure compressor 15, combustion equipment 16, a high pressure turbine 17, a low pressure turbine 18, a free power turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the core engine 11 and defines the intake 12 and nozzle 20 and a core exhaust duct 22. The engine 10 also comprises two contra-rotating propeller rotor stages 23, 24 attached to and driven by the free power turbine 19 via shaft 26. The configuration shown in FIG. 1 in which the propeller stages 23, 24 are located radially outwardly of a turbine stage 18 and perhaps slightly rearwardly of the free power turbine 19 towards the rear of the gas turbine engine 10 is termed a “pusher” configuration, as opposed to the “puller” or “tractor” configuration having the propeller stages 23, 24 towards the front of the engine 10, forward of one or both of the compressor stages 14, 15.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high pressure, low pressure and free power turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure, low pressure and free power turbines 17, 18, 19 respectively drive the high and low pressure compressors 15, 14 and the propellers 23, 24 by suitable interconnecting shafts. The propellers 23, 24, powered by the low pressure turbine 19, normally provide the majority of the propulsive thrust. In the embodiments herein described the propellers 23, 24 rotate in opposite senses so that one rotates clockwise and the other anti-clockwise around the engine's rotational axis 9.
One problem with a conventional pusher propeller gas turbine engine 10 is that its cruise speed is limited to slightly below transonic, predominantly due to the drag rise encountered when flying at higher speeds. One of the main causes of this drag rise is that generally the root of each blade forming the propeller stages 23, 24 cannot be shaped with the thin profiles required for high speed. The root has to be thick enough to guarantee the structural robustness of the blades given the high aerodynamic and mechanical loads acting on the propeller stages 23, 24. The airflow passing between the relatively thick blade roots may easily become supersonic if the propeller gas turbine engine 10 operates at transonic cruise speed, around Mach 0.8. This results in disadvantageous increased noise, aerodynamic losses and possible mechanical excitation, phenomena which it is desirable to avoid or at least limit.
Similar problems can be encountered in turbofan engines, particularly where the ratio between the bypass airflow and the core airflow is large, and where the fan comprises variable pitch blades. A turbofan gas turbine engine 410 is shown in FIG. 2 and comprises an air intake 412 and a propulsive fan 414 that generates a core airflow A and a bypass airflow B. The gas turbine engine 410 comprises, in axial flow A, an intermediate pressure compressor 416, a high pressure compressor 418, a combustor 420, a high pressure turbine 422, an intermediate pressure turbine 424, a low pressure turbine 426 and an exhaust nozzle 428. A nacelle 430 surrounds the gas turbine engine 400 and defines, in axial flow B, a bypass duct 432.
The present invention therefore seeks to address some or all of these issues.